Fibers produced using a thermoplastic polymer such as polyester or polyamide are widely used not only in clothing applications, but also in interior and vehicle interior applications, industrial applications and so on because these fibers are excellent in mechanical properties and dimensional stability. Currently, however, uses of fibers are diversified, and required characteristics thereof are accordingly diversified. Because of this, techniques that impart sensitive effects such as texture and bulkiness by the cross-section structure of fibers are proposed. In particular, “thinning of fibers” has a significant effect on the characteristics of fibers themselves and characteristics after formation of fibers into a fabric, and is a mainstream technique with regard to control of the cross-section structure of fibers.
As a method of manufacturing thin fibers, a method using so-called “sea-island” composite fiber with a sea component covering island components that form thin fibers is often employed on an industrial scale in consideration of, for example, handling characteristics in high-order processing. In that method, a plurality of island components composed of a poorly soluble component are disposed in a sea component composed of an easily soluble component, and after formation of fibers or a fiber product, the sea component is dissolved and removed to generate thin fibers composed of island components. That method is often employed as a method of manufacturing thin fibers currently produced on an industrial scale, especially microfibers and, recently, advancement of this technique has made it possible to manufacture nanofibers having a further reduced fiber diameter.
In microfibers with a single fiber diameter of several μm and nanofibers with a single fiber diameter of several hundreds nm, the surface area per weight (specific surface area) considerably increases in proportion to the square of the fiber diameter as compared to ordinary fibers (fiber diameter: several tens μm). The microfibers and nanofibers are known to exhibit a unique tactile impression created by the ductility of the fibers because the rigidity (cross-section secondary moment) of the fibers increases with the fiber diameter.
Accordingly, those fibers exhibit specific characteristics that cannot be obtained with ordinary fibers, and the fibers are being developed not only in clothing applications, but also in various applications by taking advantage of, for example, improvement of wiping performance due to an increase in contact area, gas absorbing performance associated with an ultra-specific surface area effect, and a unique soft touch.
Regarding techniques to thin fibers as described above, numerous techniques have been proposed, and among them, ultimate techniques are proposed in Japanese Patent Laid-Open Publication Nos. 2007-100243 and 2011-157646.
In JP '243, thin fibers (nanofibers) having high mechanical properties in which the toughness of (thin) fibers after dissolution of a sea component is 20 or more can be obtained by defining the fiber diameter and the average diameter and arrangement of island components in a sea-island-type composite fiber. In JP '646, the cross-section parameter of a sea-island cross-section is defined to prevent unnecessary treatment of thin fibers composed of island components at the time of dissolving and removing a sea component in a method of manufacturing thin fibers using a sea-island composite fiber. JP '243 describes that relatively high mechanical properties can be obtained, and development of the thin fibers to fiber products may be promoted.
In JP '646, it is proposed that polytrimethylene terephthalate having relatively flexible characteristics is employed in island components for improving the tactile impression and texture of a thin fiber bundle. In JP '646, thin fiber bundles and fiber products having improved softness and flexibility as compared to those in JP '243 may be obtained.
Japanese Patent Laid-Open Publication No. H05-222668 describes a sea-island composite fiber in which island components are formed such that ultra-thin fiber components of two or more types including polyamide and polyester with a size of 0.001 to 0.3 denier (equivalent to a fiber diameter of 300 nm to 6 μm) are dispersively arranged substantially without forming a group. In that technique, the sea component is removed from the sea-island composite fiber, and a heating treatment is performed so that thin fibers composed of polyester and polyamide are each uniquely shrunk. Using, for example, a shrinkage difference between the thin fibers, the alignment of the thin fibers is disordered to generate a yarn length difference in a thin fiber bundle, and in comparison with conventional thin fibers, woven/knitted fabrics having a bulky feeling in the thickness direction as well may be obtained.
In a sea-island composite fiber of conventional type as described in JP '243, thin fibers after removal of the sea component tend to form a bundle while every thin fiber is kept straight without being bent. Accordingly, the thin fibers are orderly aligned so that gaps between fibers are very small, and therefore when an external force is applied to the thin fiber bundle, the thin fibers are mostly moved in a bundle state without being opened so that exhibition of a flexible and delicate tactile impression, which is expected from reduction of the fiber diameter, may be limited. A fabric composed of such thin fiber bundles often provides a fiber product poor in water absorbency and contaminant catching performance, which require a capillary phenomenon because bulkiness in the thickness direction is hardly exhibited, and gaps between fibers are small.
As a countermeasure to this problem, the sea-island composite fiber itself may be subjected to false twist processing, or the sea-island composite fiber may be mixed with ordinary fibers composed of other type of polymer. In any case, however, the state (bulkiness or the like) of a thin fiber bundle remaining the history of an original sea-island composite fiber cross-section after removal of the sea component is not remarkably improved, development of thin fibers alone to high-performance apparels (outers, inners and the like) in which particularly the tactile impression and the texture are important and high-performance wiping cloths which are required to have wiping performance with high accuracy is difficult, and the composition design of the fabric is uselessly complicated due to, for example, mixing with ordinary fibers as described above and the configuration of a weaving and knitting composition. Thus, development of the thin fibers may be limited.
In JP '646, a fiber bundle in which thin fibers are orderly aligned is formed, and therefore the thin fiber bundle is somewhat flexible, but it is difficult to say that a flexible and delicate texture created by thin fibers is sufficiently exhibited, and in particular, the porosity between thin fibers is very small, and the problem of poor bulkiness of woven/knitted fabrics composed of the thin fibers is not solved.
In the technique in JP '668, a shrinkage difference between thin fibers generated by performing a heating treatment is used. In other words, some thin fibers exhibit a crimped structure due to shrinkage, while other thin fibers are still kept straight, and the straight thin fibers may limit the disorder of alignment in the fiber bundle.
Accordingly, that technique is not sufficient to obtain woven/knitted fabrics having bulkiness while securing flexibility specific to thin fibers, and it is strongly desired to develop a composite fiber suitable to obtain a high-performance and high-texture fiber product with a bulky feeling in the thickness direction, which is capable of maximally exhibiting flexibility specific to thin fibers and their delicate tactile impression.
It could therefore be helpful to provide a sea-island composite fiber from which a conjugate thin fiber can be manufactured with high productivity by using existing equipment, the conjugate thin fiber having various functions such as those of high-performance processing treatment and structure control in addition to mechanical properties, abrasion resistance and bulkiness while having a delicate tactile impression specific to thin fibers.